The present invention relates to low temperature molten salt liquid electrolytes. The present invention also relates to electrochemical cells comprising low temperature molten salts.
Low temperature molten salts are a class of salt compositions which are molten at low temperature. Such molten salts are mixtures of compounds (i.e. anions and cations) which are liquid at temperatures below the individual melting points of the component compounds. These mixtures, commonly referred to as "melts," can form molten compositions simultaneously upon contacting the components together or after heating and subsequent cooling.
Low temperature molten salts (ionic liquids) were used as electroplating baths by F. H. Hurley and T. P. Wier, Jr. These low temperature molten salts were obtained by combining aluminum chloride with certain alkylpyridinium halide salts, for example, N-ethylpyridinium bromide.
Some examples of low temperature haloaluminate molten salts are mixtures of aluminum chloride and N-(n-butyl) pyridinium chloride (BupyCl) or 1-ethyl-3-methyl imidazolium chloride (EMIC).
Low temperature molten salts may be used as electrolytes in electrochemical cells, batteries, capacitors, and photoelectrochemical cells. They may also be used in electroplating, electrorefining, catalysis, and synthesis.
One of the useful properties of low temperature (room temperature) haloaluminate melts is their adjustable Lewis acidity. By varying the ratio of aluminum halide to organic halide in melt, changes in solvation characteristics and electrochemical windows can be achieved. Melts that contain an excess of the organic salt are considered basic, due to the presence of halide ions that are not bound to aluminum, while melts that contain an excess of the aluminum halide component are acidic due to the presence of coordinately unsaturated species like Al.sub.2 X.sub.7.sup.-. Melts containing 50% of each component are neutral melts. The positive potential limit of a basic melt is determined by oxidation of respective halide ions. The negative limit of a basic melt corresponds to the reduction of organic cation (e.g., Bupy.sup.+, EMI.sup.+). The positive potential limits of acidic melts is due to the oxidation of the haloaluminate species to produce the respective halogen. The negative potential limits of acidic melts is due to the deposition of aluminum. Neutral low temperature melts possess an electrochemical window that corresponds to the positive limit of an acidic melt and the negative limit of a basic melt. In the case of the neutral AlCl.sub.3 -EMIC melt, the potential window is about 4.4 V.
Room temperature haloaluminate melts are very good solvents because they dissolve a wide variety of organic, inorganic, and organometallic substances. Other properties are good conductivity, negligible vapor pressure at elevated temperatures, a large electrochemical window, and good thermal stability. Therefore, they have also been used in a variety of electrochemical applications including electrodeposition and electroplating.
Impurities like water complicate electrochemical studies. Water reacts rapidly, for example, with aluminum chloride to produce protons and oxide-containing species. The reduction of Li.sup.+ was achieved from a solution of LiAlCl.sub.4 in neutral AlCl.sub.3 -EMIC. However, the plating-stripping efficiency was less than 100%, indicating an instability of the deposit.
J. Wilkes and J. M. Zaworotko have prepared the air and water stable low temperature molten salts 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF.sub.4), and EMIMeCO.sub.2. In addition, melts were prepared by others which consisted of EMI.sup.+ cation and PF.sub.6.sup.- anion. Besides room temperature melts containing EMI.sup.+ cation, melts have also been prepared with different cations, such as the 1,3 dialkylimidazolium cation and the 1,2,3 trialkylimidazolium cation. For example, 1-(n-butyl)-3-methylimidazolium cation utilizing anions such as BF.sub.4.sup.-, PF.sub.6.sup.-, and AsF.sub.6.sup.- have been prepared. The latter melts show wider electrochemical windows than EMI.sup.+ containing melts; however, they also show lower conductivity and lower melting points. In addition, the above melts are not stable toward lithium, a strong reducing agent.
Studies of stability of EMIBF.sub.4 have shown that EMIBF.sub.4 is not compatible with lithium metal even at room temperature. Within one day the originally colorless melt turns brown and after three days the melt turns into a brownish yellow gelatinous solid. In 1,2-dimethyl-3-propylimidazolium cation, the presence of the methyl group on the second carbon of the imidazolium ring results in a negative shift of reduction potential by 300 to 500 mV compared to the EMI.sup.+ cation. Compatibility studies of 1,2-dimethyl-3-propylimidazolium tetrafluoroborate have shown that this melt is more stable towards lithium than EMIBF.sub.4 at room temperature, however, at higher temperatures such as 100.degree. C. the melt turns yellow in a few minutes and dark red in a week. When cooled to ambient temperature, the melt forms a very thick liquid which is not substantially flowable. Upon continued heating at 100.degree. C., the melt turns blackish red in two weeks and appears to decompose completely within a month.
Therefore, there is a need for low temperature molten salts that are compatible with lithium metal which can be used as electrolytes in rechargeable lithium or lithium-ion batteries.